The present invention pertains to organic light active devices. More particularly, the present invention pertains to devices and methods for fabricating organic light active devices that can be used for applications such as general lighting, display backlighting, video displays, maps, digital newspapers, stereoscopic vision aides, advanced vehicle windshields, solar cells, cameras and photodetectors.
Organic light active material (“OLAM™”) makes use of the relatively recent discovery that polymers can be made to be conductors. Organic light emitting diodes (“OLED”) convert electrical energy into light, behaving as a forward biased pn junction. OLAMs can be light emitters or light detectors, depending on the material composition and the device structure. For the purpose of this disclosure, the term OLAM and OLED can be interchanged. In its basic form, an OLED is comprised of a layer of hole transport material upon which is formed a layer of electron transport material. The interface between these layers forms a heterojunction. These layers are disposed between two electrodes, with the hole transport layer being adjacent to an anode electrode and the electron transport layer being adjacent to a cathode electrode. Upon application of a voltage to the electrodes, electrons and holes are injected from the cathode electrode and the anode electrode. The electron and hole carriers recombine at the heterojunction forming excitons and emitting light.
The basic structure of an OLED display is similar to a conventional LCD, where the reactive material (in the LCD case, a liquid crystal, in the OLED case, a conjugate polymer) is sandwiched between electrodes. When an electric field is applied by the electrodes, the OLED material is brought into an excited energy state, this energy state drops down by the emission of photons, packets of light. Thus, each pixel of the OLED display can be controlled to emit light as needed to create a displayed image.
Besides attractive picture quality, an OLED display device consumes less power than liquid crystal display technologies because it emits its own light and does not need backlighting. OLED displays are thin, lightweight, and may be able to be manufactured on flexible materials such as plastic.
Unlike liquid-crystal displays, OLEDs emit light that can be viewed from any angle, similar to a television screen. As compared to LCDs, OLEDs are expected to be much less expensive to manufacture, use less power to operate, emit brighter and sharper images, and “switch” images faster, meaning that videos or animation run more smoothly.
Recently, an effort has been made to create equipment and provide services for manufacturing OLED screens. The potential OLED display market includes a wide range of electronic products such as mobile phones, personal digital assistants, digital cameras, camcorders, micro-displays, personal computers, Internet appliances and other consumer products.
There is still a need for a thin, lightweight, flexible, bright, wireless display. Such a device would be self-powered, robust, include a built-in user-input mechanism, and ideally functional as a multipurpose display device for Internet, entertainment, computer, and communication use. The discovery of the OLED phenomenon puts this goal within sight.
However, there are still some technical hurdles left to be solved before OLED displays will realize their commercial potential. OLED's light emitting materials don't have the lifespan some users may need. Presently, optimum performance in commercially viable volume production is achievable only for small screens, around 3.5 inches square or less. Storage lifetimes of at least 5 years are typically required by most consumer and business products, and operating lifetimes of >20,000 hours are relevant for most applications.
Organic-light-emitting-diode technology offers the prospect of flexible displays on plastic substrates and roll-to-roll manufacturing processes. One of the biggest challenges to the OLED display industry is from contamination by water and oxygen. The materials involved in small molecule and polymer OLEDs are vulnerable to contamination by oxygen and water vapor, which can trigger early failure.
Recently, there has been activity an developing thin, flexible displays that utilize pixels of electro-luminescent materials, such as OLEDs. Such displays do not require any back lighting since each pixel element generates its own light. Typically, the organic materials are deposited by spin-coating, vacuum deposition or evaporation. As examples, U.S. Pat. No. 6,395,328, issued to May, teaches an organic light emitting color display wherein a multi-color device is formed by depositing and patterning layers of light emissive material. U.S. Pat. No. 5,965,979, issued to Friend, et al., teaches a method of maldng a light emitting device by laminating the self-supporting components, at least one of which has a light emitting layer. U.S. Pat. No. 6,087,196, issued to Strum, et at., teaches a fabrication method for forming organic semiconductor devices using ink jet printing. U.S. Pat. 6,416,885 B1, issued to Towns et al., teaches an electro-luminescent device wherein a conductive polymer layer between an organic light emitting layer and a charge-injecting layer resists lateral spreading of charge carriers to improve the display characteristics. U.S. Pat. No. 6,420,200 issued to Yamazaki et al, teaches a method of manufacturing an electro-optical device using a relief printing or screen printing method. U.S. Pat. No. 6,402,579, issued to Pichier et al., teaches an organic light-emitting device in which a multi-layer structure is formed by DC magnetron sputtering. U.S. Pat. No. 6,422,687, issued to Jacobson, teaches an electronically addressable microencapsulated ink and display. In accordance with the teachings of this reference, microcapsules are formed with a reflective side and a light absorbing side. The microcapsules act as pixels that can be flipped between the two states, and then keep that state without ay additional power.
It is known to form an OLED layer by vacuum deposition, evaporation or spin coating. Thin layers of hole transport material and then electron transport material are formed by these known methods over a grid of anode electrodes. The anode electrodes are formed on a glass plate. A grid of cathode electrodes is then placed adjacent to the electron transport material supported by a second glass plate. Thus, the basic OLED organic stack is sandwiched between electrodes and glass plate substrates. It is generally very difficult to form the electrodes with the precise alignment needed for forming a pixilated display. This task is made even more difficult in a multicolor display, where the OLED pixels emitting, for example, red, green and blue, are formed side-by-side to fabricate a full color display. Because the OLED material and electrodes can be made transparent, it is possible to stack the color OLED pixels on top of each other, allowing for a higher pixel packing density and thus the potential for a higher resolution display. However, the electrode alignment issue still poses a problem. Typically, the well-known use of shadow masks are employed to fabricate the pixel components. Aligning the shadow masks is difficult, and requires extreme precision.
Currently, inkjet printing has gained ground as a promising fabrication method for making OLED displays. The core of this technology is very mature, and can be found in millions of computer printers around the world. However, there are some disadvantages to the adapting of inkjet printing to OLED display fabrication. It is still difficult to lay down precise layers of material using the spray heads of inkjet printers. Inkjet printing does not adequately overcome the problem of material degradation by oxygen and water vapor. Elaborate and expensive materials and fabrication processes are needed to provide adequate encapsulation of the display elements to prevent early degradation of the OLED material due to water and oxygen ingress. As an attempt to solve this contamination problem, Vitex Systems, Sunnyvale, Calif., has developed a barrier material in which a monomer vapor is deposited on a polymer substrate, and then the monomer is polymerized. A thin layer of aluminum oxide a few hundred angstroms thick is deposited on the polymerized surface. This process is repeated a number of times to form an encapsulation barrier over an OLED display. This elaborate encapsulation barrier is an example of the effort taken to prevent water and oxygen from contaminating the easily degraded OLED films that form a conventional OLED display device. It is difficult to align display pixel-sized electrodes and inkjet printed OLED material with the accuracy needed to effect a high resolution display. Electrical shorts and the destruction of pixels result from the inclusion of miniscule foreign particles, requiring the use of expensive clean room manufacturing facility for inkjet OLED fabrication. Accordingly there is a need for an improved fabrication method for forming OLED devices.